I finally reproduced the issue, using the keypad like you discovered; so
although you still didn't explain, it's probably not important at this
point what you didn't see.

I certainly would not expect this behavior, but I actually think I know the
cause. I suspect it's because Vim actually distinguishes between 1 and
<k1>, 2 and <k2>, etc. keys to allow mapping them differently. So the macro
contains special escape sequences for the keypad keys which aren't present
when using normal numerics.

To demonstrate the difference, try the following:

qao<k1><k2><k3><Esc>q
qbo123<Esc>q

Both these macros do the same thing when executed. Both will make a new
line with text, 123. But now examine the register contents:

:reg a
:reg b

For me at least, on Windows gvim, they are drastically different!

Not all systems actually send Vim a different sequence for the keypad keys;
on these systems, 1 and <k1> are not separately mappable.

I think this will only pop up when somebody tries using a recorded macro
both as a recording and as text for insertion and manipulation. I'm not
sure whether it's possible to fix without breaking the separate mapping
capability of the keypad, or even if there is something to fix at all.

A person who wrote the macro in the first place would know what they were
trying to accomplish at each step in the macro. When they pasted the
contents of the unnamed register, hoping to get a numeric value which could
then be incremented, if they saw the special escape sequences, they would
immediately know something was wrong. I don't actually think there is
anything to fix here, unless it's just seen as another place Vim's internal
representation of special keys causes weird behavior.

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Well, this would cause typing "q" to end recording to set the register
used for the next put command. I don't think that is right. When
yanking and pasting you normally do this with text, not with commands
recorded to a buffer. It may cause a problem.

Recording to the " buffer is a problem anyway, since it depends on what
buffer this turns out to be. Better not use q" unless you know what
register " refers to.

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really wish I'd listened to what my mother told me when I was young!"
"Why, what did she tell you?"
"I don't know, I didn't listen!"
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"The Hitchhiker's Guide to the Galaxy"

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In note #12 the problem was identified as something to do with the numeric
keypad.
Thus the issue is solved.

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